This invention is directed to the art of separation of entrained air and froth from pigmented coatings and the like, in a paper coater or converter. The invention particularly is directed to apparatus and method for applying clay type coatings or the like to a moving web of paper, and to apparatus and method by means of which entrained air bubbles are separated, so that a substantially air-free coating stock is applied to the web coating apparatus, and the air and air laden coating fraction is removed for recirculation to the coating supply tank.
Paper coating materials, such as pigmented coatings, are commonly applied by coater apparatus in which an excess supply of the coating is removed by a doctor blade, air knife, or the like, and recirculated to a coating supply tank. During the mixing, pumping, applying and/or doctoring steps, the coating is likely to pick up entrained air in the form of large discrete bubbles and froth which is returned to the coating tank. Due to the relatively high viscosity of the coating material commonly used for paper coating, the bubbles and entrained air particles have a tendency to remain within the coating for a long period of time. They are not readily separated merely by settling due to the slow movement of the bubbles through the liquid.
When coating materials including entrained air are redelivered to the coating head, which may be a fountain type coater, the larger bubbles may cause the coating to skip or otherwise be applied unevenly.
Various attempts have been made to break up the larger bubbles into air bubbles which are sufficiently small as to cause no problem. Thus, screens have been used to break up the larger bubbles into smaller bubbles. However, it has been found that a screen tends to compress the air on one side, and the air has a tendency to expand on the other side of the screen and recombine again into larger bubbles. It has been observed that screens break up only the very large bubbles.
Vortex type separators have been used as shown, for example, in U.S. Pat. Nos. 2,894,930 of 1958, 2,920,761 of 1960 and 3,007,542 of 1961. Such vortex separating equipment commonly requires the use of vacuum for reducing the center pressure and thereby assist in the removal of the air bubbles. While vortex separation has been successful in the separation or deaeration of paper pulp suspensions, it has not proven to be particularly successful in the separation of entrained air from highly viscous coatings. Also, the fact that the vortex separator is commonly operated at low pressure requires the imposition of a pump between the separator and the coating head. However, any time viscous clay coating is agitated, as in pumping, there is an increased danger of picking up or introducing additional air.
Other apparatus for removing or eliminating air has included elastic ultrasonic wave generators, as shown in U.S. Pat. Nos. 3,169,507, 3,169,508 and 3,169,509 of 1965.